I just did a search of the support forum for the software (SMF) used to run this forum.
The 500 server error can happen randomly when the site uses too much processor - this could be a bug in the forum software, or a matter of the server being bogged down and running out of resources.
As Greg says, the error logs don't seem to record the error 500's. There's not a lot we can do except wait for it to rear its head again.